High barrier shrink film

ABSTRACT

There is provided a heat shrinkable, high barrier laminate film which includes a hot blown, melt oriented high oxygen barrier film, of relatively high crystallinity and correspondingly relatively low ultimate elongation, laminated to a stretch oriented base film having a shrink tension substantially greater than that of said barrier film, the extent of orientation of said base film being sufficient such that its ultimate elongation is reduced so as to be commensurate with that of said barrier film. In one preferred mode, the barrier film is a layflat tubular film having its interior layflat surfaces laminated together. Representatively, the high barrier layer of the barrier film is vinylidene chloride copolymer having at least about 85% by weight vinylidene chloride content, preferably at least about 90%. Associated methods for making the laminate films are also provided.

BACKGROUND OF THE INVENTION

This invention relates generally to a laminate film suitable forpackaging, especially food packaging, and relates more particularly to aheat shrinkable laminate film having especially high oxygen barrierproperties.

In the packaging of food in thermoplastic film, it is frequentlydesirable that the film provide an oxygen barrier, i.e. have a lowpermeability to oxygen. For example, film made of a copolymer of 65 to95% by weight of vinylidene chloride and 5 to 35% of a vinyl comonomersuch as vinyl chloride, acrylonitrile or methyl acrylate, generallyreferred to as saran, is known to give a good oxygen barrier. The term"high barrier vinylidene chloride copolymer" as used herein is intendedto refer to a saran having at least about 85% by weight of vinylidenechloride. The oxygen barrier property of vinylidene chloride copolymerstends to increase with increasing content of vinylidene chloride.

Additionally, it is frequently desired in the packaging of food that thepackaging film be heat shrinkable so that the film may be shrunk snuglyabout a contained product thereby presenting an attractive packageappearance. In the manufacture of heat shrinkable film, it iscommercially advantageous to produce packaging film in tubular form,e.g. by the conventional blown bubble method, and then to stretch orientsuch film to render it heat shrinkable by the conventional trappedbubble method. In many cases, such film is irradiatively crosslinkedbefore orientation to enhance orientability. Representatively, suchprocesses are disclosed in U.S. Pat. No. 3,741,253 issued June 26, 1973to Brax et al.

The term "high barrier" film as conventionally used refers to an oxygenbarrier thermoplastic material composed predominately of the barrierconstituent to the extent that the material is brittle and relativelydifficult to stretch orient. Generally, the higher the barrierconstituent, the higher the barrier property, but the higher the degreeof crystallinity which tends to embrittle the film to impartcorresponding low impact strength.

One approach to making a high barrier, heat shrinkable multilayer filmhas been to melt-form the composite film and then to stretch orient thefilm before crystallization of the high barrier material proceeds to anadverse extent. An alternative approach provides for lamination ofstretch-oriented barrier film to a heat shrinkable, base film, i.e. thehigh barrier film is stretch oriented after melt forming and beforecrystallization proceeds to an adverse extent. Apparently, stretchorientation of nascent high barrier material reduces the size ofcrystalline regions of the microstructure so that the material does notbecome brittle. In this alternative approach, it would be desirable toeliminate the step of stretch orienting the nascent high barrier film.This is the problem with which the present invention is concerned.

Of general interest are the disclosures of U.K. Pat. No. 1,591,423 for"Laminate Film of Polyethylene and Vinylidene Chloride Copolymer" andU.K. Pat. No. 1,591,424 for "Process of Heating Food in a Package ofPolymeric Laminate Films" by Thompson published June 24, 1981, whichdisclose a laminate film formed by coating vinylidene chloridecopolymer, other than by melt extrusion, onto the surfaces of a heatshrinkable film.

SUMMARY OF THE INVENTION

It is a primary object of the invention to provide a high oxygen barrierlaminate film that is heat shrinkable in spite of the use of relativelybrittle high barrier thermoplastic material. It is a secondary object toprovide for redundant barrier plies in an exemplary embodiment.

Accordingly, there is provided a heat shrinkable, high barrier laminatefilm which includes a hot blown, melt oriented high oxygen barrier filmlaminated to a stretch oriented base film having a heat shrink tensionsubstantially greater than that of said barrier film, the ultimateelongation of said barrier film being substantially less than that ofsaid base film before orientation and the extent of orientation of saidbase film being sufficient such that its ultimate elongation is reducedso as not to substantially exceed that of said barrier film, andpreferably is commensurate therewith. In one preferred mode, the barrierfilm is a layflat tubular film having its interior layflat surfaceslaminated together by self-welding. Preferably, the barrier ply of thebarrier film is vinylidene chloride copolymer having a vinylidenechloride content of at least about 85% by weight, most preferably atleast about 90%.

Thus, by selective stretch orientation of the base film, trouble-somestretch orientation of the high barrier film is avoided.

The term "melt oriented" is intended to refer to a relatively low extentof orientation that is imparted to a hot blown film incidental to thehot blowing formative process. The relatively low extent of orientationis manifested by a low heat shrink tension relative to that obtainedwith stretch orientation.

The term "high barrier" is intended to refer to oxygen barrierthermoplastic copolymeric material composed predominately of the barrierconstituent to such an extent that the material is brittle due to a highdegree of crystallinity which is manifested by the material having lowimpact strength relative to that of the material when stretch oriented.

Additionally, there is provided a method for making a heat shrinkable,high barrier laminate film which includes the steps of forming a hotblown, melt oriented high oxygen barrier film; stretch orienting a basefilm further provided that said barrier film has an ultimate elongationsubstantially less than that of said base film before orienting and saidbase film after orienting has a heat shrink tension substantiallygreater than that of said barrier film, the extent of orientation beingsufficient such that its ultimate elongation is reduced so as not tosubstantially exceed that of said barrier film and preferably iscommensurate therewith; and then laminating said barrier film to saidbase film. Preferably, the method further includes providing the barrierfilm as a layflat tubular film and laminating together its interiorlayflat surfaces by self-welding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details are given below with reference to the drawings wherein:

FIG. 1 schmatically depicts a cross-sectional view of one preferredlaminate film of the invention wherein a melt oriented layflat tubularhigh barrier film is laminated to a stretch oriented high shrinkablebase film;

FIG. 1A schematically depicts a transverse cross-section of analternative laminate film wherein such layflat high barrier film islaminated between heat shrinkable base films;

FIG. 2 illustrates the multiply structures of the foregoing embodiments;and

FIG. 3 schematically depicts a process for making the foregoingembodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, there is shown a preferred high barrier, heat shrinkable film10 of the invention wherein a layflat tubular barrier film 11 islaminated to a heat shrinkable planar base film 12. It is a requiredfeature of the invention that barrier film 11 be a hot blown filmthereby being melt oriented, i.e. having low heat shrinking potentialrelative to that typical of stretch orientation as in base film 12. Thelayflat tubular film 11 is joined together at its internal layflatsurfaces as indicated at interface 13. One external layflat surface islaminated to heat shrinkable component film 12 at interface 14.Optionally, the surface of heat shrinkable component film 12 that isjoined to the layflat tube may be printed with labeling matter so thatupon lamination a trapped label results. The multilayer structure oflayflat film 11 includes an internal oxygen barrier layer of highbarrier material, as further discussed below. The internal layflatsurfaces of layflat film 11 are of a polymeric material of the typecharacterized as self-welding so that upon passing layflat tube 11through a set of heated press rollers, self-welding of the internallayflat surfaces occurs to form heat sealed interface 13. The externalsurface 15 of layflat tube 11 is of a polymeric material that isrendered pressure sensitive adherable to compatible polymeric surfacesby electrostatic conditioning, such as conventional corona dischargetreatment. A representative thickness of the tubular film is about onemil overall. A representative thickness of the heat shrinkable componentfilm is about 0.5-0.75 mil.

Heat shrinkable base film 12 provides a stretch-oriented heat shrinkablesubstrate upon which hot blown layflat tube 11 is laminated at interface14. Heat shrinkable film 12 has an interior layer that initiates heatshrinking and a surface layer at interface 14 that preferably isrendered pressure sensitive adherable to the outer surface 15 of layflattube 11 by electrostatic conditioning. In a less preferred mode,lamination at interface 14 can be accomplished by application of asuitable conventional adhesive. In general, lamination at interface 14is conducted without the application of heat so as not to cause anysignificant preshrinkage of heat shrinkable base film 12. The exposedsurface 16 of base film 12 is of a polymeric material that is preferablyheat sealable to the exterior surface 15 of layflat tube 11, so that thelaminate film may be used in form/fill/seal applications wherein alongitudinal lap heat seal is made to define a product containingenvelope within the enclosure of the film.

Summarizing the surface requirements of the various components oflaminate film 10, the internal layflat surfaces of the layflat tube areof a self-welding polymeric material, the external surface of thelayflat tube is preferably responsive to corona discharge treatment andis heat-sealable to one surface of the heat shrinkable substrate, andthe other surface of the heat shrinkable substrate is preferablyresponsive to corona discharge treatment.

The laminate film 10 may be characterized as heat shrinkable since thestretch oriented base film 12 will initiate heat shrinkage of thelaminate film upon elevation of the laminate film to the appropriateshrink temperature and provides substantial shrink tension. The hotblown high barrier layflat tubular film, having relatively low shrinktension, will follow the heat shrinkage of base film 12 in compatiblefashion. It is a required feature of the invention that the base film bestretch-oriented to a selected extent such that its ultimate elongationproperty in the oriented condition is about equal to or less than theultimate elongation property of the melt-oriented barrier film, andpreferably is commensurate therewith. It is believed that this featureaccounts for the nontransference of the relatively brittle nature of thehigh barrier film into the composite film. The term "ultimateelongation" is used in the conventional sense to refer to percentelongation at tensile failure. For example, a typical heat-shrinkablefilm of irradiatively crosslinked polyethylene being stretch oriented toan elongation of about 400-500% will have an ultimate elongationcapacity of about 90%, which is commensurate with the ultimateelongation capacity of hot blown, melt oriented high barrier saran film.The shrink tension of the base film will be substantially greater thanthat of the barrier film and therefore will be shrink controlling. Thus,by selective stretch orientation of the base film, troublesome stretchorientation of the high barrier film is avoided.

The laminate film 10 is characterized as high barrier film according totwo aspects. First, a high oxygen barrier material, as defined above, isused such as vinylidene chloride copolymer having a relatively highvinylidene chloride content, representatively about 85% by weight orgreater and preferably at least about 90%. Additionally, the multilayerstructure of layflat tube 11 has an internal oxygen barrier layer sothat upon collapsing tube 11 to the layflat configuration two separatespaced apart barrier layers will result in the layflat configuration,one barrier layer appearing above interface 13 at the self-weldedinternal layflat surfaces and one barrier layer appearing belowinterface 13, in the regions indicated by 17 and 18 respectively. Thetwo separate spaced apart barrier layers provide redundant barriersagainst minor manufacturing defects such as pin holes, carbon particleinclusions, and the like as may randomly occur in a large scalemanufacturing process.

In a less preferred embodiment of the invention, the barrier film may bea planar film similar in all respects to the foregoing except that, forexample, the tubular barrier film 11 is slit into two mirror imagefilms, each of which are laminated into a composite structure as above.Of course, in this configuration redundant barrier plies will not bepresent since, in effect, only one side of the slit tubular film isbeing used.

In FIG. 1A, there is shown an alternative embodiment relative to that ofFIG. 1 being similar in all respects but having a second heat shrinkablebase film 12a adhered to the other of the exterior layflat surfaces oflayflat tube 11. Base films 12 and 12a are identical in terms ofmultilayer structure and are arranged so that they sandwich layflat tube11 and so that their multilayered structures are in mirror imagesymmetry about layflat tube 11. Interfaces 14 and 14a are preferablyresponsive to corona discharge treatment thereby providing the means bywhich heat shrinkable films 12 and 12a are laminated to the exteriorlayflat surfaces of layflat tube 11, respectively. Further, the exposedsurfaces of shrinkable films 12 and 12a as indicated at 16 and 16a areself-welding to facilitate use of the laminate film in form/fill/sealapplications as discussed above.

In FIG. 2, there is shown an exploded cross-section through theforegoing embodiments of the multiply structure 20 making up thelaminate film. The layflat tube 11 has a mirror image multiply structureabout interface 13, shown in exploded configuration, which is theinterface between the interior layflat surfaces of layflat tube 11.Layers 21 and 21a are interior tacky coatings that promote self-weldingof the interior layflat surfaces of tube 11. A preferred composition isEVA (ethylenevinyl acetate copolymer) having 10-40% VA (vinyl acetate)content by weight. Layers 22 and 22a are structural layers andpreferably are composed of polyethylene, most preferably low density orlinear low density. Optionally, in layflat tubular film 11 layers 21 and21a may be deleted with the further requirement that layers 22 and 22abe of a self-welding polymeric material such as polyethylene. Layers 24and 24a are barrier layers of a high oxygen barrier material such as VDC(vinylidene chloride) copolymer having a relatively high VDC content,representatively at least about 85% by weight, most preferably about90%. As discussed above, the invention provides for impartingsubstantial heat shrinkable properties to an otherwise relativelybrittle high barrier film. Layers 23 and 23a are adhesive interlayersthat tie the barrier layers to adjacent layers, respectively. Apreferred adhesive is EVA having 5-40% VA content orethylene-methacrylate copolymer. Layers 25 and 25a form the exteriorlayflat surfaces of layflat tube 11 and preferably are composed ofpolyethylene or EVA having a VA content in the range of about 4-10%.Optionally, a conventional opacifying pigment may be added to thislayer.

Heat shrinkable base film 12 is joined to one face of the layflat tubeat interface 14. Thus, layer 26 is preferably of a material that isrendered pressure sensitive adherable to layer 25 by corona conditioningand preferably is composed of EVA having 4-10% VA content. Layer 27 is aselectively oriented shrink controlling layer and is preferablypolyethylene, most preferably low or medium density, that has beencross-linked with ionizing radiation prior to orientation. Exposedsurface layer 28 of heat shrinkable base film 12 is heat sealable tolayer 25a at surface 15 of layflat tube 11. Preferably, layer 28 ispreferably composed of the same polymeric material as layer 26.Optionally, layers 26 and 28 may be deleted so that component film 12 iscomposed simply of a single layer of heat shrinkable material 27, suchas polyethylene, so long as film 12 is adherable to the layflat tube atinterface 14 and is heat sealable to the exterior surface of the layflattube at surface 15. The discussion of FIG. 2 thus far describes themultilayer structure making up the embodiment shown in FIG. 1.

The multilayer structure making up the embodiment of FIG. 1A furtherincludes a second heat shrinkable film 12a which is substantiallyidentical to heat shrinkable film 12 and is joined in the structure atsurface 15 and is disposed so that film 12a lies in mirror imagesymmetry relative to film 12 insofar as layer 26a corresponds to 26,layer 27a corresponds to 27, and layer 28a corresponds to 28. The basicmultiply structure of the embodiment of FIG. 1A may be represented inminimal terms by the notation S//C/B/A//A/B/C//S wherein A is theself-welding interior surface layer of the layflat tube, B is anintermediate barrier layer, C is a heat sealable and corona treatableexterior layer of the layflat tube, and S is a single ply heatshrinkable planar film.

Key properties of the material of the invention are determinable by thewidely accepted testing procedures of ASTM. Specifically, theorientation and shrink tension properties are determinable by ASTMD2838. The ultimate elongation property is determinable by ASTM D882.The oxygen barrier property is determinable by ASTM D3985. The impactstrength property is determinable by ASTM D3420. The extent ofcross-linking property is determinable by ASTM D2765.

In FIG. 3, there is depicted a preferred process line 30 for making theembodiment of FIG. 1. The process begins with the provision of roll 31of layflat tubular film of the type described above. The layflat film isfed out at 32 and passes through the nip of heated press rollers 33, theheated pressing causing self-welding of the interior layflat surfaces ofthe layflat tube. Optionally, the starting material may be providedalready in this condition when the hot blowing formative film process isconducted as part of the overall process by passing the nascent hotblown film through nip rollers while the film is still hot. One exteriorsurface of the layflat tube is then treated by corona discharge withconventional corona discharge unit 34 to render one face of the layflattube pressure sensitive adherable. Roll 35 provides a supply of heatshrinkable base film which is fed out at 36 to conventional coronadischarge unit 37 which treats one face of the heat shrinkable film torender it pressure sensitive adherable. The two films are then merged inthe nip of press rollers 38 with their corona treated surfacescontacting one another so that at 39 a laminated film results of thetype shown in FIG. 1. Takeup roll 40 then accumulates the laminate film.The embodiment shown in FIG. 1A may be made in similar fashion exceptthat both exterior layflat surfaces of the layflat tube are coronatreated and components 35 through 37 are duplicated on the other side ofthe process line to sandwich the layflat tube between mirror image heatshrinkable films, the sandwiching being completed in the nip of rollers38.

In use, the laminate film of the invention may be incorporated in chubpackaging of sausage. Chub packaging is essentially conventionalform/fill/seal packaging wherein film is first formed into a tube with alongitudinal overlap seal made to close the side of the tube. One end ofthe formed tube is then clipped, a flowable meat product is then stuffedinto the tube, and the remaining end of the tube is clipped therebycompleting the food log. Then the chub package is heat shrunk byimmersion in a hot water bath at a temperature within the shrinktemperature range of the film, whereupon the film shrinks snugly aboutthe contained food product.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be utilized without departing from the principles andscope of the invention, as those skilled in the art will readilyunderstand. Accordingly, such modifications and variations may bepracticed within the scope of the following claims.

What is claimed is:
 1. A heat shrinkable, high barrier laminate filmcomprising:a hot blown, melt oriented high oxygen barrier film includinga layer of vinylidene chloride copolymer laminated to a stretch orientedbase film including a layer of polyethylene having a heat shrink tensionsubstantially greater than that of said barrier film, the ultimateelongation of said barrier film being substantially less than that ofsaid base film before orientation and the extent of orientation of saidbase film being sufficient such that its ultimate elongation is reducedso as not to substantially exceed that of said barrier film.
 2. Thelaminate film of claim 1 wherein the ultimate elongation of said basefilm is commensurate with that of said barrier film.
 3. The laminatefilm of claim 1 wherein said barrier film is a layflat tubular filmhaving its interior layflat surfaces laminated together.
 4. The laminatefilm of claim 3 wherein said interior layflat surfaces are self-weldedand said exterior layflat surface and the adjacent surface of said basefilm are corona conditioned.
 5. The laminate film of claim 3 wherein theouter surface of said base film is heat sealable to the exterior surfaceof said layflat film.
 6. The laminate film of claim 3 wherein said basefilm comprising irradiatively crosslinked and oriented polyethylene andsaid layflat film comprises inside and outside surface layers ofpolyethylene and an internal layer of vinylidene chloride copolymerhaving at least about 85% vinylidene chloride content.
 7. The laminatefilm of claim 3 further comprising a heat shrinkable mirror image basefilm laminated to said layflat film, said base films sandwiching saidlayflat film and being disposed in mirror image symmetry.